(1) Field of the Invention
The present invention relates to a process in which a faujasite zeolite, especially a faujasite zeolite having a high silica/alumina ratio, can be easily prepared. More particularly, it relates to a process for the preparation of a faujasite zeolite, which comprises heating and crystallizing a reaction mixture comprising a silica source, an alumina source, and an alkali metal source, wherein a transparent faujasite germ solution obtained by mixing and aging an aqueous solution of an alkali metal aluminate, an aqueous solution of an alkali metal silicate, and an aqueous solution of an alkali metal hydroxide is made present in the reaction mixture.
(2) Description of the Related Art
A faujasite zeolite ordinarily has an oxide molar composition represented by the following formula: EQU 0.9.+-.0.2M.sub.2 O.A1.sub.2 O.sub.3.xSiO.sub.2.wH.sub.2 O
wherein M represents an alkali metal cation, x is a number of from 2.5 to 6, and w is a number of from 6 to 9.
Faujasite zeolites are used as a cracking catalyst for refining petroleum in the field of the petrochemical industry or the like, and as an adsorptive separating agent for use in the separation of paraxylene or the like.
Ordinarily, these catalysts or adsorptive separating agents are required to have high solid acid strength, good heat resistance and acid resistance. In the case of a faujasite zeolite, the SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio has a close relationship to these properties, and in a faujasite zeolite having a higher SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio these properties are excellent.
A faujasite zeolite having a relatively low silica ratio, that is, an SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio of 2.5 to 4, has a poor heat resistance and acid resistance because of this low silica molar ratio. Therefore, faujasite zeolites having an SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio of at least 4 are widely used industrially and high-silica faujasite zeolites having an SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio of at least 4.5 are especially desired. However, it has been considered difficult to synthesize faujasite zeolites having an SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio of at least 4.5, especially at least 5. In the synthesis of such faujasite zeolites, the intended zeolites are produced only under very restricted conditions. Accordingly, the kinds and quantitative ratios of starting materials should be strictly and carefully selected and the reaction should be carefully carried out while strictly controlling such conditions as the mixing state, the degree of aging, the reaction temperature, and the reaction time. These difficulties are increased particularly when it is intended to prepare a faujasite zeolite having a high purity.
Various processes have heretofore been proposed for preparing high-silica faujasite zeolites. According to a typical process disclosed in Japanese Examined Patent Publication No. 36-1639 or No. 42-16941, a silica source such as silica sol, silica gel, or finely divided solid silicic acid, sodium aluminate and sodium hydroxide are used as the starting materials and these starting materials are reacted under strictly controlled conditions to form a faujasite zeolite having an SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio of at least 4. In this process, however, it is indispensable that expensive amorphous solid silica should be used as the silica source, and after the mixing of the silica source, alumina source and alkali metal source, aging should be conducted for a long time, and crystallization then conducted at a high temperature. Therefore, a faujasite zeolite cannot be produced at a low cost.
Furthermore, a fatal defect is encountered when it is intended to synthesize a faujasite zeolite on an industrial scale with a good reproducibility. Namely, it is impossible to carry out the reaction while stirring and mixing the reaction system. Especially, when stirring is effected at the step of converting an amorphous reaction product of an aqueous gel, formed through long-time aging after mixing of the starting materials, to a faujasite zeolite, crystallization to the intended faujasite zeolite is inhibited and the majority of the product is converted to an aluminosilicate mineral of no practical utility such as naturally occurring phillipsite mineral analogues (hereinafter referred to as "phillipsite") and gmelinite. Accordingly, in order to synthesize a high-silica faujasite zeolite having a high purity, it is indispensable that the synthesis should be carried out while the reaction mixture in the form of an aqueous gel is allowed to stand, and mixing under stirring should be avoided. However, if this synthesis process under the stationary condition is carried out on an industrial scale, stirring required to effect the heat transfer for maintaining the reaction temperature is not conducted, and thus the heat transfer in the reaction system becomes difficult and the temperature becomes uneven, resulting in a disadvantage such as the simultaneous formation of phillipsite and gmelinite. Various means are adopted for avoiding the simultaneous formation of phillipsite and gmelinite in the above-mentioned stationary synthesis process, but the problem of the simultaneous formation of phillipsite and gmelinite has not been completely solved.
Moreover, as means for reducing the manufacturing cost of a high-silica faujasite zeolite, there have been proposed various improved processes. For example, Japanese Examined Patent Publication No. 47-4866 discloses a process for preparing a faujasite zeolite having an SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio of at least 4 by using as the silica source cheap sodium silicate which has been considered to be unable to give a high-silica faujasite zeolite, wherein an amorphous nucleus-forming core substance (seed) which has been prepared in advance and has an average particle size of 0.01 to 0.1 .mu.m is added to a reaction mixture comprising sodium silicate, sodium aluminate, and aluminum-sulfate.
Moreover, Japanese Examined Patent Publication No. 53-33556 discloses a process for preparing a faujasite zeolite having an SiO.sub.2 /A1.sub.2 O.sub.3 molar ratio of 4.5, wherein a zeolite seed prepared in advance from an aqueous mixture of sodium silicate and sodium aluminate is added to a mixture of silica and alumina sources.
In each of these known processes, specific means such as the addition of an amorphous or crystalline seed is adopted so as to obtain a high-silica faujasite zeolite from a cheap silica source such as sodium silicate. However, synthesis under stirring, which will make large-scale production possible, cannot be realized in these known processes, though the manufacturing cost has been reduced in these improved processes by using a cheap silica source.